1. Field of the Invention
The present invention relates to an image forming apparatus including a detection unit which detects a formed pattern.
2. Description of the Related Art
Multicolor image forming apparatuses which form a multicolor image by superimposing a plurality of colors (for example, yellow, magenta, cyan, and black) are becoming popular. The multicolor image forming apparatus forms a multicolor image by superimposing a plurality of colors. If the image forming positions of the respective colors shift from ideal positions, the image quality degrades. To reduce the image forming positional error, color misalignment correction patterns are formed by image forming units of the respective colors and read to compute the color misalignment amounts of the respective colors. The image forming positions are then corrected in accordance with the computed color misalignment amounts.
The correction pattern can be detected by an optical sensor or the like arranged near an intermediate transfer member. More specifically, the pattern is recognized by irradiating the intermediate transfer member with light emitted by a light emitting element, and detecting a difference between the quantity of light reflected by the surface of the intermediate transfer member and that of light reflected by the correction pattern formed on the intermediate transfer member. Japanese Patent Laid-Open No. 2003-177578 proposes a specular reflection light detection method as a method of detecting reflection light.
Repetitive image formation changes the surface state of the intermediate transfer member, decreasing the gloss. This is because the surface of the intermediate transfer member wears upon cleaning of toner left on the surface of the intermediate transfer member and rubbing of the intermediate transfer member with an intermediate transfer mechanism (transfer device). If the quantity of specular reflection light from the surface of the intermediate transfer member changes, the difference between the quantity of specular reflection light from the surface of the intermediate transfer member and that of specular reflection light from the correction pattern transferred onto the intermediate transfer member cannot be detected.
The inventor considered a method of irradiating a correction pattern with light from two light sources and detecting the specular reflection light and diffuse reflection light by one light receiving unit as a method of detecting a correction pattern on a poor-gloss intermediate transfer member. However, this method has a problem in which if the optical axis of the specular reflection light emitting unit and that of the diffuse reflection light emitting unit deviate from desired design values, a mutual detection error is generated. The mutual detection error is a difference between the time when light emitted by the specular reflection light emitting unit reaches the light receiving unit and the time when light emitted by the diffuse reflection light emitting unit reaches the light receiving unit. The mutual detection error changes the correction pattern detection result from an original detection result, finally decreasing the color misalignment correction precision. To suppress the decrease in color misalignment correction precision, the difference (mutual detection error) between the correction pattern detection timing by specular reflection light and that by diffuse reflection light needs to be specified.